1. Technical Field
The disclosed embodiments relate to internal loopback testing and calibration of RF transceivers.
2. Background Information
Both the transmitter and the receiver of a cellular telephone are ideally linear devices that introduce minimal distortion into the signal being communicated. One type of distortion is referred to as second-order distortion. A linear amplifier generally introduces only a small amount of second order distortion when the amplifier is operating at a low output power level. As the output power increases, however, the output power at the fundamental frequency (the frequency of the input signal) rises at a first rate with respect to overall rising output power, whereas the output power due to second-order distortion rises at a faster rate. When the output power of the amplifier is high enough, the output power of the second-order distortion reaches the output power of the fundamental signal. This point of intersection is referred to as the second order intercept point (IP2). The IP2 point of a system, such as a cellular telephone transmit chain or a cellular telephone receive chain, can be used as a measure of the second-order distortion of the system.
One way to measure the IP2 of a system involves using so-called two-tone analysis. A signal of one pure frequency is referred to as a “tone”. Two tones of equal strength but different frequencies are put through the system. The system will generate an output at each of the two fundamental frequencies, but will also generate an output at other frequencies due to second-order effects. The outputs due to second-order effects will include, for example, an output that has a frequency equal to the sum of the frequencies of the two input tones. The outputs due to second-order effects will also include, for example, an output that has a frequency equal to the difference of the frequencies of the two input tones. The output powers of the output signals that are not at either of the two fundamental frequencies are measured and used to determine the IP2 of the system.
To enhance the operation of the transceiver within a cellular telephone, it is often desired to measure the IP2 of a transceiver and then to calibrate various parts of the transceiver so as to reduce the IP2 exhibited by the transceiver. External signal sources can be used to generate the signals of the two tones for use in the two-tone analysis described above, but such external sources may only be available in the factory during factory calibration. Although such factory calibration may allow the cellular telephone transceiver to be calibrated to account for variations in the semiconductor fabrication process used to make transceiver integrated circuits, such factory calibration cannot account for performance changes that occur due to temperature changes that occur during operation of the cellular telephone. Similarly, such factory calibration cannot account for performance changes that occur due to voltage supply variations that occur during operation of the cellular telephone transceiver. It is therefore desired to be able to monitor IP2 and to calibrate parts of the transceiver during use of the cellular telephone outside the factory such that distortion can remain minimized as operating conditions change.
Several ways have been proposed for using the transmitter of a transceiver to generate the two tones needed for a two-tone IP2 analysis test so that the IP2 measurements and calibrations can be made outside of the factory in a functioning transceiver. One suggestion is set forth in the paper entitled “An IP2 Improvement Technique for Zero-IF Down-Converters” by Darabi et al. This paper describes a long loop approach whereby an external power amplifier (PA) and low-noise amplifier (LNA) are used to generate one tone blocker with AM modulation in every slot. This approach, however, has several drawbacks.
First, an unnecessarily large amount of power that even may exceed a maximum output power rating of the transceiver can be driven back onto the transceiver's antenna during calibration. Usually the power level of the blocker used in calibration testing is higher than the power level of the blocker specified in standards to detect a nonlinear effect. Second, the one tone blocker approach utilizes an operating external power amplifier that generally consumes more power than is necessary. The resulting increased power consumption can reduce talk time due to the power amplifier being turned on in every slot. Third, the one tone blocker approach is not an efficient way to detect the modulated signal in an OFDMA modem.
A second approach is set forth in the WiFi, IEEE 802.11 arts in a paper entitled “A Single-Chip Digitally Calibrated 5.15-5.825-GHz 0.18-um CMOS Transceiver for 802.11a Wireless LAN,” by Bouras et al. The Bouras et al. paper suggests using an on-chip loopback connection to generate one tone for IQ mismatch calibration. If this approach were extended and applied to IP2 calibration of a cellular telephone receiver, several problems would likely occur. First, the loopback circuitry operates in a voltage driven mode. The baseband signal to be detected in the receiver would therefore likely be of an undesirably small amplitude due to the long on-chip conductors that often carry the high frequency RF loopback signals from the transmitter to the receiver. Often the distance between transmitter and receiver within a cellular telephone transceiver integrated circuit is substantial in order to prevent coupling between the receiver and transmitter. This substantial distance means that if the internal loopback connection technique were employed, then the transmitter would have to drive through long conductors to supply the two tones to the receiver circuitry for internal loopback calibration. As a result, the baseband signal as received at the receiver would likely be of such an undesirably small amplitude that calibrating the receiver would be difficult or impossible. Second, the circuits in the loopback path of the WiFi circuit might generate nonlinearities such as intermodulation terms and harmonics. These nonlinearities may interfere with receiver calibration of a cellular telephone transceiver.